1. Field of the Invention
The present invention relates to a corrosion resistant tensile member, e.g. a metal strip, for use underground in mechanically stabilized earth (MSE) structures, such as retaining walls for bridges. More specifically, the invention provides a protective coating for controlled corrosion resistance of the tensile member and methods to construct a protectively coated corrosion resistant tensile member.
2. Description of Related Art
Retaining structures employing reinforced soil techniques such as retaining walls, bridge abutments, sea walls, revetments, steep slopes, etc. employ various types of reinforcement. One type of retaining structure, a mechanically stabilized embankment (MSE), provides a specific means of constructing retaining structure.
An MSE structure is typically formed by embedding reinforcements in granular soils at specific vertical and horizontal spacing. These reinforcements usually take the form of strips, grids, or ladders and can be made from various engineering plastics, metal (such as steel), engineering fabrics, or other types of materials. As the embankment is constructed, a portion of the stresses in the soil are transferred to the reinforcements by friction, bearing, passive resistance, or a combination of these mechanisms.
A key problem with the use of a metal as the tensile member, even a galvanized metal, is that the metal corrodes over time. Even further, with the MSE structures of the type described, the tensile member (e.g. metal plate, rod, bar, beam or the like), is embedded in the earth structure, with one end of the tensile member configured for external connection to other components, such as facings. In order to connect to the other components, the tensile member may be structurally compromised, usually by a deformity such as a bolt hole, weld, etc. The tensile member as a whole is subject to corrosion over time, by virtue of its composition; and the region compromised also corrodes at the same rate as the regions that have not been compromised. The region of structural compromise can be a region of weakness from the center of the deformity to a distal end of the tensile member. The region of structural compromise can be a region of weakness from the center of the deformity in equidistant and opposing linear spans of the tensile member. The region can further be from the center of the deformity in non-equidistant and opposing linear spans of the tensile member.
The corrosion of the tensile members can occur over time due to the electrical conductivity of the earth (which causes loss of metal due to electrolysis), the presence of minute quantities of air (which causes oxidation of metal), the presence of water (which enhances electrolysis), and the presence of salts (which enhances electrolysis). The tensile members are engineered to have a given useful lifetime, which depends on the particular application for each tensile member (e.g., retaining wall for a bridge, retaining wall for a sidewalk, sewage pier, etc.) In general, a larger (i.e., wider and thicker) tensile member is used when long-term stability is required. The problems of corrosion of the tensile member have been addressed in the past as follows.
U.S. Pat. No. 4,710,062 addresses the reinforcement of a rolled metal strip for use in stabilized earth structures. The rolled metal strip is thickened at periodic intervals along its length during formation of the strip. The strip is cut into required lengths such that each strip length has an end reinforced region through which an aperture is then formed to receive a bolt passing thorough a bracket of a facing. The strip may include transverse ribs at intervals on both faces of the strip to assist engagement with the surrounding soil. However, this configuration merely thickens the strip, and does not delay an onset of corrosion at the deformity.
There is a need in the art for a tensile member used in MSE structures that addresses the corrosion proximate the structurally compromised or weakened region of the tensile member relative to a remainder of the tensile member, particularly in the MSE structure.